In small bowl, with mixer at high speed, beat egg whites with cream of tartar until stiff peaks form. Stir egg yolks into cheese sauce; then, with rubber spatula, gently fold in whites, one-third at a time, just until blended.

Pour mixture into souffle dish. If desired, with back of spoon, make 1-inch-deep indentation all around in soufflé mixture, 1 inch from edge so center will puff up like a top hat when soufflé is done. Bake soufflé 40 to 45 minutes until knife inserted under top hat comes out clean, loosely covering soufflé with foil after 40 minutes if top begins to brown too much. Serve immediately.